A concentrated winding system brushless DC motor has been proposed for reducing torque fluctuation (See Patent Document 1, for example). The brushless DC motor is designed to reduce torque fluctuation by the shape of tip portions of teeth portions on the inner diameter side of a stator core. The tip portion is provided with a cut surface on the inner peripheral surface of the tooth portion up to an opening which excludes a tooth body.
A motor using a rotor with buried permanent magnets therein has also been proposed (See Patent Document 2, for example). The motor using a rotor with buried permanent magnets therein is designed to achieve effective torque generation by preventing the short circuit of the magnetic flux at the both ends of a permanent magnet, and thereby allowing a stator to receive the magnetic flux even in the end portions of the permanent magnet. The motor using a rotor with buried permanent magnets therein is formed to include holes to prevent the short circuit of magnetic flux in the vicinity of the outer surface of a rotor core. The holes are formed in contact with the end portions of permanent magnet burying holes and the buried permanent magnets therein.
A permanent magnet rotating electric machine has also been proposed for solving noise issues (See Patent Document 3, for example). The permanent magnet rotating electric machine is designed to reduce harmonic magnetic flux by approximating the waveform of induced electromotive force to a sine wave without losing the effective magnetic flux (fundamental wave magnetic flux) of permanent magnets. The permanent magnet rotating electric machine has a stator with concentrated armature winding, and a rotor with permanent magnets embedded in a plurality of permanent magnet insert holes formed in a rotor core. The permanent magnet rotating electric machine is formed as follows. The rotor includes a multiple number of slits formed in the rotor core on the outer peripheral side of the permanent magnets. The slits are elongated from the inner peripheral side to the outer peripheral side of the rotor. The width of the slit in the circumferential direction is made narrower on the outer peripheral side of the rotor core than the inner peripheral side. The slits are spaced so that a distance between adjacent slits is made narrower on the outer peripheral side of the rotor core than on the inner peripheral side. A recess is then formed between poles on the outer peripheral side of the rotor core so that a magnetic pole angle of the rotor core is in a range of between 90 and 120 degrees of electric angle.    Patent Document 1: JP 3301978    Patent Document 2: JP 11-98731A    Patent Document 3: JP 2005-27422A